In many cases, parts such as gears, pulleys, and shafts for motor vehicles and industrial machinery are manufactured by roughly shaping them by hot forging or cold forging, by subjecting them to cutting work and thereafter to casehardening by carburizing quenching or carbo-nitriding quenching. Unfortunately, if pre-quenched austenite grains are coarsened by the heat from carburizing or carbo-nitriding, there easily arise problems that the fatigue strength as a part decreases and that the amount of deformation at the quenching time increases.
Generally, it has been thought that, as compared with cold-forged parts, in hot-forged parts, the austenite grains are less liable to be coarsened at the time of carburizing or carbo-nitriding. In recent years, however, with the progress of hot forging technique, hot forging has frequently been performed in various temperature ranges, and the number of hot-forged parts with the austenite grains coarsened at the time of carburizing or carbo-nitriding has increased. Therefore, there has been demanded a hot-rolled steel bar or wire rod in which austenite grains can be stably prevented from being coarsened at the heating time in the process of carburizing or carbo-nitriding even if hot forging is performed in various temperature ranges, and techniques concerning steels and/or producing methods therefor have been proposed in Patent Documents 1 to 3.
Specifically, Patent Document 1 discloses a “Grain stabilized carburizing steel” in which a steel with limited amounts of sol.Al and N and a limited ratio of “sol.Al/N” is heated to a temperature of 1200° C. or higher and thereafter is hot worked.
Patent Document 2 discloses a “Producing method of steel having superior cold workability and preventing coarsening of grain during carburization heating” in which the Al/N ratio and the “Al+2N” amount are limited, and further the amount of AlN precipitated in a rolled material and the ferrite grain size number are defined. As seen in the title of invention and the object of invention of Patent Document 2, the technique proposed in Patent Document 2 is premised on the fact that the steel is roughly formed as rolled by cold working, and subsequently is subjected to carburizing treatment.
Patent Document 3 discloses a “Case hardening steel excellent in preventability of coarse grain and its producing method” in which the amount of AlN precipitated, the bainite structure fraction, the ferrite band, and the like are defined. As described in the paragraph [0002] of Patent Document 3, the technique proposed in Patent Document 3 is also premised on the fact that the steel is roughly formed by cold forging, and subsequently is subjected to carburizing quenching    [Patent Document 1] JP56-75551A    [Patent Document 2] JP61-261427A    [Patent Document 3] JP11-106866A